Question: If $3a + 9b = -6$ and $3x + 4y = -10$, what is $45b - 12x + 15a - 16y$ ?
Solution: $= 15a + 45b - 12x - 16y$ $= (5) \cdot (3a + 9b) + (-4) \cdot (3x + 4y)$ $= (5) \cdot (-6) + (-4) \cdot (-10)$ $= -30 + 40$ $= 10$